Instrumentation FundamentalsInstrumentation Fundamentals

Module 1 – Pressure ScalesModule 1 – Pressure Scales

Units of Pressure

Pressure Scales & Conversions

Atmospheric, PSIG, PSIA, PSID, Bar

Manometers

Pressure Pressure

This module will cover:

• The physics of pressure– Units of measure (SI, Metric, Imperial)– Pressure scales and conversions

• How pressure is measured– Elastic elements (bourdon, bellows diaphragm)– Electrical elements (strain gauge, piezoelectric)– Sensors, Switches and Transmitters

What is it?

Pressure is an operating parameter that is relevant in many applications. (similar to voltage in an electric circuit)

• Pressure applied over a given area can be used for useful work.– Steam pressure, Water pressure

• Pressures can be measured to infer the condition of other process parameters.– Flow, level

The Physics of PressureThe Physics of Pressure

Pressure is defined as “force per unit area”

Force

AreaPressure =

Therefore any object or material having a weight will exert a pressure over the area the force is acting on.

The Units of PressureThe Units of Pressure

Pressure is defined as “force per unit area”

Force

AreaPressure =

Common units include:

Pounds per Square Inch (psi)

KiloPascals (kPa)

Pound force, Kilogram force

Newton, dyne

Square Inches, Square feet

Square Centimeters, Square Meters

Example of pressure from a 1 cubic Example of pressure from a 1 cubic foot pound force acting on a surfacefoot pound force acting on a surface

1 cubic foot of copper

550 lbs

144 in2

= 3.8 psi

1 cubic foot of lead

708 lbs

144 in2

= 4.9 psi

Each base has an area of 144 in2

1 cubic foot of water

62.4 lbs

144 in2

= 0.43 psi

1 cubic foot of mercury

849 lbs

144 in2

= 5.9 psi

550 lbs 708 lbs 62.4 lbs 849 lbs

More Pressure ScalesMore Pressure ScalesPSI and kPa are the most common pressure

scales but there a few more:

• Inches* of water • Inches* of mercury• Bar• Atmos• Torr (vacuum)

* or millimeters when using metric

Example of the various pressure scalesExample of the various pressure scales

27.6806“H20

Applied process

pressure is

1 psi or

6.89 kPa

2.03602“Hg

0.068947 Bar

The same process pressure is being applied to each gauge. Each gauge has a different scale calibration.

0.068046 Atmos

The choice of scales will depend on

• the amount of pressure being measured (high pressure = psi/kPa, low pressure = inches H20)

• The type of application ( flow = inches H20, blood pressure = inches of Hg.)

Conversion FactorsConversion FactorsNeed to Know: psi and kPa conversion

Imperial vs Metric vs SI• 1 cubic foot of water that weighs 62.4 lbs acting over an

area of 144 in2 produces a pressure of 0.433 pound per square inch (psi)

• The same volume of water weighs 28.3 Kilograms over an area of 929 cm2, therefore the pressure is 0.03 kilograms per square centimeter. (30.46 g/cm2)

• SI use Newton per sq. meter and call it the Pascal 1 psi = 0.006894757 Pascals = 6.895 kiloPascals

Need to KnowNeed to Know

Ball Parking:1 psi ≈ 7 kPa3 psi ≈ 21 kPa15 psi ≈ 105 kPa20 psi ≈ 140 kPa

3 to 15 psi is a common pressure range20 kPa to 100 kPa is also a common pressure

range

Inches of Water ScaleInches of Water Scale

This scale is used to measure small pressures.

The properties of water are known and constant and can be used as a primary standard.

12 inches of water exerts a pressure of 0.433 psi

Pressure is proportion to the height of the water column (hydrostatic head pressure)

Water ColumnWater ColumnThe hydrostatic head produced by

a column of liquid is proportional to the height and density of the liquid.

0.433psi

12 “ H20

P = height x DensityP = height x Density

(Density = Mass/Volume)(Density = Mass/Volume)

Density of water is 0.0361 lbs/in3

P = 12 x 0.0361

= 0.433 lbs/in2

Water ColumnWater Column

The greater the height the greater the hydrostatic head.

0.866psi

24 “ H20

P = height x DensityP = height x Density

(Density = Mass/Volume)(Density = Mass/Volume)

Density of water is 0.0361 lbs/in3

P = 24 x 0.0361

= 0.866 lbs/in2

ManometersManometers can be used as a primary standard to

measure small pressures

Atmospheric Pressure

Applied Process Pressure Atmospheric Press

U-Tube Manometer

Height (h) of displaced water = applied pressure

h

Reading pressure with a U-tube Manometer

If the total displacement h = 3“ the applied pressure would be 3”H20 = 3”WC = 0.108 psi

Applied Process Pressure Atmospheric Press

Height (h) of displaced water = applied pressure

h

Using Mercury as a filling liquid increases the pressure range by 13.6 times.

Well Type ManometersWell Type ManometersThe well type uses one measuring arm. Gives a

larger pressure range

Mercury filled well type manometers can measure up to 30 psi and more. (6 footer)

Can be used as a primary standard.

Inclined Plane ManometerInclined Plane Manometer

Used for very small pressure measurements. Very sensitive, often used to measure room pressures.

Gauge Pressure (psig)Gauge Pressure (psig)

The standard pressure measurement is referenced to atmospheric pressure and is called gauge pressure.

The scale units on the manometer could be calibrated in

– inches of water (gauge)– inches of mercury (gauge)– psig

And all measurements would be relative to atmospheric pressure 14.7 psi

(varies slightly with elevation and weather)

psi in atmosphere

Gauge, Absolute and Atmospheric PressureGauge, Absolute and Atmospheric Pressure

Any pressure above atmosphere is called gauge pressure (psig)Any pressure below atmosphere is a vacuum (negative gauge pressure)Absolute pressure (psia) is measured from a perfect vacuum

Differential Pressure (psid) has no reference to either absolute vacuum or atmospheric pressure

Gauge Pressure (psig)Gauge Pressure (psig)

Applying 1 psi would produce a displacement of about 2 in. Hg or 30 in. H2O

Since the reference side of the manometer is open to atmosphere, the applied pressure would be read as gauge pressure

i.e. 1psig or just 1 psi

1 psi Atmos

h

Standard GaugeStandard Gauge

Some gauges may not include Some gauges may not include the “g” after psi, some will.the “g” after psi, some will.

• When a gauge has no input applied, it will read 0 psig

• The pressure range for this gauge is 0 – 100 psi

• What is the range in kPa?

Pressure Range & ScalePressure Range & Scale

This gauge has a pressure range of 0 to 30 in. H2O

The pressure being measured is still gauge pressure.

What is the maximum psig that can be applied? kPa?

Small pressure measurementsSmall pressure measurements

Dwyer differential pressure gauge registers a differential of 0 - 2 psi, 1/8" npt. High and low pressure input ports on side and back. Manual set point. Max 15 psi and 140ºF. 4-3/4" diameter x 2" high.

Examples of psig, psia and vacuumExamples of psig, psia and vacuum

20 psig = 20 + 14.7 = 34.7 psia

60 psia

= 60 – 14.7 = 45.3 psig

10 psia

= 10 – 14.7 = -4.7 psig

= -4.7 = - 9.6 ”Hg

0.0361 x13.6

PSIA – Absolute PressurePSIA – Absolute Pressure

A gauge with a psia scale will indicate 14.7 when no pressure is applied.

The compound gauge is more common than psia, it measures vacuum and gauge pressure.

This gauge has a range of 0 – 30 in. Hg

vacuum and 0 to 15 psig

Differential Pressure (psid)Differential Pressure (psid)

Differential pressure = 4 psid

h = 4

0.0361 x 13.6= 8.15 inches of H20 differential

6 psi 2 psi

Differential Pressure Gauge (psid)Differential Pressure Gauge (psid)

Requires 2 inputs.

Must observe pressure polarity, i.e. hi side / lo side

Differential Pressure Cell TransmitterDifferential Pressure Cell Transmitter

The d/p cell is often used to measure level and flow.

What is the maximum allowable input pressure in psi?

0 – 200 in. H20

Typical input range

Differential Input

4 – 20 mA output

2 wire transmitter

Pressure Conversion ChartPressure Conversion ChartPressure Units psi kPa inches of Hg inches of H20 atmospheres bar

psi 1 6.894 2.036 27.681 .0681 .06895

kPa 0.1450 1 .2953 4.0147 .009669 .01

inches of Hg 0.4912 3.3864 1 13.595 .03342 .03386

inches of H2O 0.03613 .2491 .07355 1 .002458 .002491

atmospheres 14.696 101.33 29.92 406.8 1 1.0133

bar 14.504 100 29.53 401.86 .9869 1

Ball ParkingBall Parking

1 psi = 7 kPa

1 inch Hg = 0.5 psi

100 inch H20 = 3.5 psi

1 Bar = 1 Atmos = 14.7 psi

AccurateAccurate

1 psi = 6.89 kPa

1 inch Hg = 0.49 psi

100 inch H20 = 3.61 psi

1 Bar = 14.5 psi = 100 kPa

Exercise (ball park is fine)Exercise (ball park is fine)

What is this in psi, kpa, inches of H20?

What is this in psig, psia, inches of Hg?